In recent years, wireless local area network (LAN) has found wide practical application. Building on IEEE 802.11n, which are wireless LAN standards, IEEE 802.11ac standards have been established by the Institute of Electrical and Electronics Engineers, Inc. (IEEE). At present, standardization activities for IEEE 802.11ax have been started as a successor to IEEE 802.11n/ac. In 802.11ax Task Group (TG), unlike conventional wireless LAN standards, an improvement in user throughput per wireless terminal apparatus, as well as an improvement in peak throughput, is given as a fundamental requirement, in consideration of a scenario that Access Points (AP) or stations (STA) are densely arranged. For the improvement in user throughput, the introduction of a high-efficiency simultaneous multiplex transmission scheme (access scheme) is indispensable.
In standards up to and including IEEE 802.11ac, an access scheme is employed that is an autonomous distribution control scheme, which is referred to as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as an access scheme. In the CSMA/CA, the opportunity of the wireless terminal apparatus (the AP, the STA, or the like) to perform transmission is temporally divided using a carrier detection process referred to as a Carrier Sense (CS). For example, the wireless terminal apparatus can determine, using a threshold referred to as the Carrier Sense Level, that, in a case where a power higher than a Carrier Sense Level is detected, the radio resource is busy, and that, in a case where only a power lower than the Carrier Sense Level is detected, the radio resource is idle. The Carrier Sense Level is also referred to as a Clear Channel Assessment Level (CCA level), a CCA Threshold, or the like.
For the efficient use of the radio resource, in IEEE 802.11ax TG, a system has been under study in which every wireless terminal apparatus makes the CCA level variable based on information (a reception level (Received Signal Strength Indication (RSSI) of a signal received from the AP that is connected), the number of times that a transmission error occurs, or the like) relating to the reliability of data communication (NPL 1 and NPL 2). If the CCA level is raised, because the opportunity of the wireless terminal apparatus to determine that the radio resource is busy occurs less frequently, it is considered that the transmission opportunity (TXOP) can be efficiently ensured even under the environment where the APs or the STAs are densely arranged. Because the raising of the CCA level increases the number of wireless terminal apparatuses that obtains the TXOP in the same radio resource, there is a problem that the number of interference signals increases, but it is expected that communication quality is maintained with adaptive modulation transmission or the like.